Wafer alignment in a general semiconductor manufacturing apparatus will be described with reference to FIGS. 11, 12, and 13. If a wafer W is supplied to the semiconductor manufacturing apparatus, first, mechanical pre-alignment is performed in step S01. A mechanical alignment apparatus MA mechanically aligns the wafer W by using the periphery of the wafer W and an orientation flat or notch (notch N is shown in FIG. 12) formed on the periphery of the wafer to determine the rough position of the wafer W. The mechanical alignment precision is about 20 μm.
In step S02, the wafer W is set on a chuck CH and supplied to a stage STG by a wafer supply apparatus (not shown), and pre-aligned in step S03. In pre-alignment, a mirror MM is inserted into an optical path in a scope SC (the mirror MM moves to the right in FIG. 13). Light from an alignment mark illumination light source Li is detected by a sensor S1 set to a low magnification. Left and right pre-alignment marks (PAL and PAR) shown in FIG. 12 are detected using the low-magnification sensor S1, and their mark positions are obtained to attain the center of the wafer. The alignment precision in this pre-alignment is about 4 μm.
Then, global alignment is performed to accurately obtain the position of the wafer W and the position of an exposure shot in step S04. In global alignment, the mirror MM is removed from the optical path (the mirror MM moves to the left in FIG. 13). A sensor S2 set to a high magnification is used to measure the positions of a plurality of global alignment marks (FX1 to FX4 and FY1 to FY4) on the wafer W shown in FIG. 12. In this manner, X- and Y-direction shifts of the wafer W, the rotational component, and the magnification component of the shot array are obtained. The global alignment precision must be 50 nm or less in a machine which manufactures current 256-Mbit memories. After global alignment ends, exposure starts in step S05.
As described above, accurately obtaining the wafer position requires at least pre-alignment and global alignment on the chuck CH. Furthermore, since pre-alignment and global alignment have different detection targets, two kinds of marks are required.
In pre-alignment, the mark must be detected in a wide field of view after mechanical rough alignment. The mark must be detected by a low-magnification scope and must be as large as about 100 μm. In global alignment, the mark is precisely detected by a high-magnification scope because the mark has already been aligned with a shift of about 4 μm by pre-alignment. Hence, the marks are small.
In recent semiconductor fabrication, the wafer processing called CMP (Chemical Mechanical Polish) is mainly performed. An alignment mark on a wafer having undergone the CMP must be accurately measured. Hence, the shape and line width of the alignment mark must be tuned in accordance with the processing. Both the pre-alignment mark and global alignment mark must be respectively tuned. Tuning of the two kinds of marks requires a long time, and decreases the yield.
Recently, in order to minimize the manufacturing cost of the semiconductor, a scribe line in which the alignment mark and the like can be arranged is narrowed. In some cases, a scribe line with a width equal to or smaller than the size of the pre-alignment mark is required.
Additionally, in the step of forming a bonding pad, since the steps of a scribe line become large, the size of the alignment mark is further strictly limited. FIGS. 14A to 14D show the scribe line, a pre-alignment mark PA, and a global alignment mark F. Reference numerals s11 and s12 in FIG. 14A denote edges of the scribe line, and an interval between the edges is the width of the scribe line. When the steps of the scribe line are small as shown in FIG. 14B, no problem arises in the relationship between the width of the scribe line and the size of the alignment marks shown in FIG. 14A.
However, in the step of forming the bonding pad, the steps of the scribe line become large, and a resist is applied to these large steps. Hence, unobservable areas are increased, and the areas in which the pre-alignment marks can be accurately observed are further narrowed. Hence, the pre-alignment marks of 100 μm are difficult to detect. FIGS. 14C and 14D show this state. When the steps of the edge portions s11 and s12 become large and a resist R is applied to the steps, the resist thickness sharply changes at the edge portions. When the change portions are observed with a scope SC shown in FIG. 13, the intensity of scattered light on the resist change portions is increased, and the areas of DA1 and DA2 in FIG. 14C cannot be observed by dark field illumination. As a consequence, the edges of the pre-alignment mark PA cannot be observed. When bright field illumination is used, light in the resist change portions is also scattered and the intensity of reflected light is decreased. Therefore, in an image observed by the scope SC, the areas DA1 and DA2 shown in FIG. 14C cannot be observed, and thus the edges of the pre-alignment mark PA become black and cannot be observed.
While the detection of the marks becomes difficult, demands are arising for short-time detection and measurement. Since the number of wafers processed per unit time must be increased, the time of processing called alignment not accompanied by exposure must be shortened as much as possible.
As described above, in general alignment, the following problems arise.
(1) Two kinds of marks are required for wafer alignment.
(2) Since the scribe line becomes narrow, or the observable area becomes narrow, a large pre-alignment mark cannot be arranged in the scribe line.
(3) Alignment processing time is required to be shortened.